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Linking synoptic weather types to daily rainfall in Auckland

This paper provides a probabilistic analysis of the linkage between synoptic weather types and the daily rainfall in Auckland, New Zealand for the period 1962-2008. Under each synoptic type and for each season, the daily rainfall data from the Auckland Airport site are examined and the probability of occurrence of high rainfall events calculated. The effects of different weather types on the chance of having a wet day (with daily rainfall > 1 mm) are modelled using logistic regression and taking into the account of the effect of El Niño Southern Oscillation (ENSO) and Interdecadal Pacific Oscillation (IPO). The results show that the synoptic weather types can be meaningfully linked to the rainfall data, conforming well to general meteorological principles. Relatively dry (low rainfall) conditions are associated with anticyclonic weather types and relatively wet (high rainfall) conditions are related to cyclonic states. However, under the influence of the same synoptic type, the average rainfall level and the chance of high rainfall events in Auckland to some degree vary across seasons. Two cyclonic states, associated with a low system centred to the east or west of the South Island, are found to provide much higher probabilities of a wet day in Auckland than other weather types. A change in the phase of ENSO, as indicated by the phase of the Southern Oscillation Index (SOI), is also shown to change the probability of a wet day in Auckland. Compared to the neutral SOI phase, on average, the low (high) SOI phase is associated with a 7% (9%) decrease (increase) in the odds of having a wet day. The effect of the IPO is found to be statistically insignificant in this study, with only a 2% decrease in the odds during the positive IPO phase relative to the negative IPO phase. This analysis demonstrates the utility of a synoptic climatological method for environmental studies in the Auckland context.